Phosphors are currently used in many important devices such as fluorescent lamps, RGB (red, green, blue) screens, lasers, and crystal scintillators for radiation detectors, radiographic imaging and nuclear spectroscopy. Perhaps the most important property of any phosphor is its brightness, i.e. its quantum efficiency, which is the ratio of the number of photons emitted by the phosphor to the number of photons absorbed. Other important properties include the spectral region of maximum emission (which should match commonly-used photodetectors), optical absorption (minimum self-absorption is desired), decay time of the emission (for some applications fast is desired), and the density. Additionally, phosphors may be categorized as either intrinsic, when the luminescence originates from the host material, or extrinsic, when impurities or dopants in the host material give rise to the luminescence.
In general, superior scintillators exhibit high quantum efficiency, good linearity of the spectral emission with respect to incident energy, high density, fast decay time, minimal self-absorption, and high effective Z-number (the probability of photoelectric absorption is approximately proportional to Z5). Specific scintillator applications determine the choice of phosphor. For example, scintillators used for active and passive radiation detection require high density, and brightness, whereas scintillators used for radiographic imaging also require fast decay time.
An exceptionally good scintillator is cerium-activated lutetium oxyorthosilicate. This material has been conveniently abbreviated in the art as either LSO:Ce or Ce:LSO. LSO:Ce is a crystalline solid that includes a host lattice of lutetium oxyorthosilicate (Lu2SiO5, abbreviated LSO) that is activated by a small amount of the rare-earth metal dopant cerium (Ce). Cerium is an excellent activator because both its 4f ground and 5d excited states lie within the band gap of about 6 eV of the host LSO lattice. LSO:Ce is very bright, i.e. it has a very high quantum efficiency. LSO:Ce also has a high density (7.4 gm/cm3), a fast decay time (about 40 nanoseconds), a band emission maximum near 420 nanometers, and minimal self-absorption.
Oxyorthosilicate scintillators, including LSO:Ce, have been documented in the following reports and patents.
“Czochralski Growth of Rare-Earth Orthosilicates (Ln2SiO5)” by Brandle et al (Journal of Crystal Growth, vol. 79, p. 308-315, 1986), incorporated by reference herein, describes yttrium oxyorthosilicate (YSO) activated with Ce, Pr, Nd, Sm, Gd, Tb, Er, Tm, or Yb.
“Single-Crystal Rare-Earth doped Yttrium Orthosilicate Phosphors” by Shmulovich et al. (Journal of the Electrochemical. Society:Solid-State Science and Technology, vol. 135, no. 12, p. 3141-3151, 1988), incorporated by reference herein, describes single crystals of rare-earth activated YSO (prepared according to aforementioned Brandle et al.) that include a green phosphor containing YSO activated with Tb and Gd, and a red phosphor containing YSO activated with Tb and Eu.
“Czochralski Growth of Rare Earth Oxyorthosilicate Single Crystals” by Melcher et al. (Journal of Crystal Growth, vol. 128, p. 1001-1005, 1993), incorporated by reference herein, describes the Czochralski preparation of single crystals of GSO:Ce, LSO:Ce, and YSO:Ce.
“Czochralski Growth and Characterization of (Lu1−xGdx)2SiO5” by Loutts et al. (Journal of Crystal Growth, vol. 174, p. 331-336, 1997), incorporated by reference herein, describes the preparation and properties of single crystals of cerium-activated oxyorthosilicates having a crystal lattice of lutetium and gadolinium.
U.S. Pat. No. 4,647,781 to Takagi et al. entitled “Gamma Ray Detector,” which issued on Mar. 3, 1987, incorporated by reference herein, describes a cerium-activated oxyorthosilicate scintillator having the general formula Gd2(1−x−y)Ln2xCe2ySiO5 wherein Ln is yttrium and/or lanthanum, wherein 0≦x≦0.5, and wherein 1×10−3≦y≦0.1.
U.S. Pat. No. 4,958,080 to Melcher entitled “Lutetium Orthosilicate Single Crystal Scintillator Detector,” which issued on Sep. 18, 1990, describes an x-ray detector employing a transparent, single crystal of cerium-activated lutetium oxyorthosilicate (LSO:Ce).
U.S. Pat. No. 5,264,154 to Akiyama et al. entitled “Single Crystal Scintillator,” which issued on Nov. 23, 1993, incorporated by reference herein, describes a single crystal cerium-activated oxyorthosilicate scintillator having the general formula Gd2−(x+y)LnxCeySiO5 wherein Ln is Sc, Tb, Lu, Dy, Ho, Er, Tm, or Yb, wherein 0.03≦x≦1.9, and wherein 0.001≦y≦0.2.
U.S. Pat. No. 6,689,298 to McClellan et al. entitled “Crystalline Rare-Earth Activated Oxyorthosilicate Phosphor,” which issued on Feb. 10, 2004, incorporated by reference herein, describes a variety of single crystal phosphors such as lutetium yttrium phosphor (host lattice LYSO), lutetium gadolinium phosphor (host lattice LGSO), and gadolinium yttrium phosphor (host lattice GYSO) that have been doped with rare earth dopants Sm, Tb, Tm, Eu, Yb, and Pr.
Other exceptionally good scintillators include rare earth doped lanthanum halides, such as cerium doped lanthanum fluoride, cerium doped lanthanum chloride, cerium doped lanthanum bromide, and cerium doped lanthanum mixed halides.
While the scintillator properties of LSO:Ce are exceptional, high-quality single crystals are difficult and expensive to prepare. The high cost, which is at least partly due to the high cost of starting materials (high purity Lu2O3 powder) and equipment (iridium crucibles for containing the Lu2O3 powder that melts at about 2150 degrees Celsius), and the tendency of the crystal boule to form cracks that limit the amount of usable single crystal from the boule, limits efforts to develop other types of crystals with an LSO host lattice.
High Purity Germanium (HPGe) detectors allow for the resolution of closely spaced peaks in a gamma-ray energy spectrum, and at this level of resolution each element has a distinctive spectrum. The number of gamma rays observed is proportional to the product of the total detector efficiency and the counting time. If counting time is limited, large detector mass is needed to achieve good statistical accuracy.
A large, inexpensive, ambient temperature gamma-ray detector with the energy resolution of current HPGe detectors would greatly simplify the task of finding and identifying the isotopic composition of radiation sources.